Lime has been found to have many uses. Such uses have included the treatment of sewage sludge to remove pathogens, lime stabilization of waste water, pasteurization of sludge and many other treatment processes.
Examples of such processes involving use of lime include U.S. Pat. Nos. 5,013,458; 5,186,840; 5,405,536; 5,554,279 and 5,681,481.
In many uses, lime in the form of calcium oxide (CaO) is mixed with water (H2O) to form calcium hydroxide (CaOH2). The chemical reaction which occurs during such mixing, gives off heat in the form of an exothermic reaction, and when done with excess water is commonly referred to as “lime slaking”, a process which is accomplished in a device known as a lime slaker. The resulting mixture of lime (CaOH2) and water is known as a lime slurry. Lime slurries are known to have some unique properties, one of which is its inherent ability to form scale on surfaces which come into contact with the lime slurry. The formation of scale, or “scaling” can render the various delivery systems, such as pipes, troughs, conduits, etc. unusable over time due to the build-up of scale. Alternatively, such scale build-up can require frequent cleaning and descaling of the equipment that is used to handle lime slurries. Cleaning and descaling operations can be considerably labor intensive.
Lime slurries also contain a certain amount of inert material that is commonly referred to as grit. Grit results because the calcium oxide, generally in the form of quicklime (CaO) contains a certain amount of material other than CaO. Typically, lime contains approximately 90%-95% CaO, and 5%-10% inert material, or grit.
The presence of grit in a lime slurry can cause numerous problems, including grit build-up in downstream process chambers or vessels, grit acting to plug or clog nozzles and orifices through which the slurry passes, and abrasions caused to treatment equipment and slurry delivery equipment such as pumps, pipes, valves, etc. because of the abrasive nature of grit particles.
Various techniques have been attempted for separation of unwanted grit that is contained within a lime slurry.
For example, U.S. Pat. No. 4,482,528 describes some of the problems that are encountered when trying to separate unwanted grit from a lime slurry. For example, using a gravity classifier to separate grit from a lime slurry when discharging from a lime slaker, is described as being unable to effectively remove particles smaller than 100 mesh (140 microns).
The gravity separation of grit, directly after the slaking of lime in a lime slaking device can be problematic, in that the amount and size of the grit removed will vary as the rate of discharge from the lime slaker varies. At high discharge rates from the slaker, the slurry will have less retention time in the grit separation chamber, which can be insufficient for the smaller grit particles to settle, such that the grit particles can be conveyed through the separation chamber and into the finished lime slurry, resulting in inconsistent and highly variable amounts of grit removal.
U.S. Pat. No. 4,482,528 attempts to improve upon gravity separation of grit by the use of a cyclone followed by at least one gravity classifier device. The cyclone operates centrifugally, and removes grit and lime, which discharges from the cyclone into a gravity classifier, wherein dilution water is added in order to allow the grit to settle in the classifier.